This invention relates to an exhaust system for an outboard motor, and more particularly to an improved exhaust system that is compact in nature and yet which will ensure that water cannot flow back into the critical portions of the exhaust system and/or engine, particularly under rapid deceleration of the associated watercraft.
It is well-known that the design of exhaust systems for outboard motors present a number of difficulties. In the first instance, because of the desire for maintaining a compact nature for outboard motors, it is difficult to ensure that there will be adequate treatment of the exhaust gases within the space available. This problem is particularly acute when it is also desired to provide a catalyst system for treating the exhaust gases before they are discharged to the atmosphere.
Conventionally, it has been the practice in outboard motor construction to employ a relatively large expansion chamber that is contained within the drive shaft housing of the engine. Exhaust gases are past from this expansion chamber to the atmosphere primarily through an underwater high-speed exhaust gas discharge in the lower unit. Normally exhaust gases are delivered to the expansion chamber from an exhaust pipe that extends downwardly from the exhaust ports of the engine in the associated power head and which terminates in the expansion chamber.
As is well-known, at least a substantial portion of the lower unit and at times the drive shaft housing may be submerged in the body of water in which the watercraft is operating. This deep degree of submersion exists at times when the associated watercraft is being propelled at low speeds or, alternatively, during idling. When this condition occurs, the water from the body of water in which the watercraft is operating can enter the expansion chamber through the underwater exhaust gas discharge and, at times, may reach the level of the exhaust pipe. This presents certain problems, particularly when engines having two-cycle engines are employed because, as it is well-known, the pressure at the end of the exhaust pipe can, at times, be less than atmospheric. As a result, there is a danger that water in the expansion chamber can actually be drawn back into the exhaust ports of the engine when such negative pressures exist.
It is, therefore, a principal object of this invention to provide an improved exhaust system for an outboard motor that will ensure against water entering the exhaust ports of the engine through the exhaust pipe.
It is a further object of this invention to provide an improved exhaust system for an outboard motor employing a trap between the expansion chamber and the underwater exhaust gas discharge so as to ensure against the likelihood of significant amounts of water being entered into the expansion chamber.
The problems aforenoted are even more difficult when it is proposed to employ a catalyst in the exhaust system for treating the exhaust gases before they are released to the atmosphere. As is well-known, it is desirable to place the catalyst as close to the engine exhaust ports as possible so as to ensure a proper temperature for operation. However, if water is able to enter the exhaust system and come into contact with the catalyst, then not only can the exhaust purification be substantially reduced, but the catalyst bed itself may become damaged or permanently disabled. Because of its high temperature and the fact that normally a ceramic-type of catalyst material is employed, any water coming into contact with the hot catalyst can very well shatter the catalyst bed.
It is, therefore, a still further object of this invention to provide an improved exhaust system for an outboard motor wherein a catalyst is employed but the catalyst is protected from contact with water entering through the exhaust system.
It is a further object of this invention to provide an improved catalytic exhaust system for an outboard motor.
Generally, it has been the practice to position substantially all of the components for the exhaust system of an outboard motor in the drive shaft housing and lower unit. The only components normally provided in the power head are the exhaust manifold and the upper connection of the exhaust pipe to the exhaust manifold, with the exhaust pipe depending into the expansion chamber in the drive shaft housing. At times, catalytic devices may be positioned in either the exhaust manifold or in the exhaust pipe. However, these types of systems are prone to the aforenoted defects.
The inventors have discovered that a number of the problems in connection with the prior art types of devices can be employed by positioning portions of the exhaust system in the power head in addition to the exhaust manifold of the engine. By so positioning the components of the exhaust system in the power head, a trap section can be formed so as to reduce the likelihood that water can enter the expansion chamber, even under extreme conditions such as abrupt decelerations. However, because of the extremely compact nature of outboard motors, and particularly their power heads, it had been previously thought impossible to position any substantial portion of the exhaust system or exhaust treatment devices within the power head.
It is, therefore, a still further object of this invention to provide an improved arrangement for permitting certain components of the exhaust system and exhaust treatment to be positioned in the power head apart from the engine but still maintain a compact power head construction.